Direct gamma-spectrometric measurement of the 226Ra 186.2 keV line for detecting 238U/226Ra disequilibrium in determining the environmental dose rate for the luminescence dating of sediments

In the age determination of sediments according to the luminescence method, it is important to know whether there was secular equilibrium in the Th and U decay series during accumulation of the environmental radiation dose in geological times. This can be investigated via gamma-ray spectrometry, whereby—in the 238U series—214Pb and 214Bi are considered as the indicators for 226Ra-mobility, and 210Pb as the indicator for 226Rn-emanation. In the present work, the direct measurement of 226Ra—which can give extra information on the interpretation of the radioactive equilibrium—is investigated as to its precision and accuracy. Since there is a serious spectral interference on its 186.2 keV gamma-line (by 235U at 185.7 keV), various correction procedures were outlined and their performance was checked via the analysis of (certified) reference materials, leading to the recommendation of a “method of choice”.